The present invention relates, in general, to methods and apparatus for boiler flame diagnostics and control. More particularly, the present invention provides methods and apparatus for monitoring the operating state of burner flames using temporal irreversibility and symbol sequence techniques.
Economic pressures and increasingly restrictive environmental regulations have contributed to an increasing need for advanced management systems that efficiently regulate utility boilers. Inefficient boiler control is responsible for wasting large amounts of fuel heating value and releasing nitrogen oxide pollutants into the atmosphere.
Monitoring systems that accurately reflect burner-operating states are essential to advanced boiler management. Accurate monitoring of burner-operating states is more important for advanced low-NOx burners than conventional burners because low-NOx burners are more sensitive to changes in operating parameters and feed system variations. Conventional combustion monitoring systems provide information that has been averaged over many burners and long time scales (e.g., measurements of excess air, coal feed, or NOx emissions at time scales of several minutes or hours). However, large NOx and carbon burnout fluctuations can occur in individual burners over short time scales (i.e., between about 10 seconds to fractions of a second). These fluctuations produce widely different boiler performance for operating conditions that otherwise are indistinguishable. Accordingly, combustion diagnostics should reflect both long and short time-scale transients for more reliable boiler optimization.
A key variable in the combustion of fossil fuels, such as oil, gas and pulverized coal, is the air/fuel (xe2x80x9cA/Fxe2x80x9d) ratio. The A/F ratio strongly influences the efficiency of fuel usage and the emissions produced during the combustion process (especially, for low-NOx burners). The A/F ratio also affects slagging, fouling and corrosion phenomena that typically occur in the combustion zone. In current steam generators fired with fossil fuel, the A/F ratio is controlled by measurement of oxygen and/or carbon monoxide (xe2x80x9cCOxe2x80x9d) concentration in the stack gases or at the economizer outlet. In either case, the gas measurement is taken at a location removed from the actual location of the combustion process. Unfortunately, in multiburner, steam generator furnaces the A/F ratio differs from burner to burner and accordingly may vary significantly with burner location. Since both combustion efficiency and NOx generation levels depend on the localized values of the A/F ratio (i.e., the distribution and mixing within each flame), measurement and control of the global A/F ratio produced by the entire furnace of the steam generator does not necessarily optimize performance.
A number of factors can change the A/F ratio during normal boiler operation. These variables include coal pulverizer wear, which may lead to a change in the size distribution of the coal particles, change in the overall fuel flow rate from the pulverizer, change in the distribution among burners of the fuel flow, change in the distribution of fuel within the flame due to erosion/corrosion of the impeller or conical diffuser, change in the overall air flow rate change in the distribution of air among individual burners and change in the distribution of air among individual burners due to change in the position of air registers.
All burners (especially, burners with staged air and/or fuel injection) undergo characteristic transitions in dynamic stability (i.e., bifurcations) as the above parameters are varied. The most important burner bifurcations are caused by the nonlinear dependence of flame speed on the relative amounts of fuel and air present. In particular, flame speed (i.e., combustion rate) drops exponentially to zero when the A/F ratio approaches either fuel-lean or fuel-rich flammability limits. Fuel-lean refers to conditions where excess air (i.e., oxygen) is present and fuel-rich refers to conditions where excess fuel is present. Local variation in the A/F ratio creates some zones adjacent to the burner that sustain combustion and other zones that do not sustain combustion. These zones may interact through complex mechanisms that depend on the details of turbulent mixing imposed by burner design, specific operating settings and the relative amounts and spatial distribution of incoming fuel and air. In coal-fired burners, the complexity of the process is further increased by the presence of both solids and volatile components in the fuel, which mix and burn at characteristically different rates. The details of the distribution and interaction of combusting and non-combusting zones is critical in determining the efficiency of fuel conversion and the levels of pollutants emitted (such as oxides of nitrogen and carbon monoxide).
Although the dynamics of coal-fired burners are complex, certain global bifurcations in flame structure typically occur. These global bifurcations represent conditions under which the dominant structure of the flame (e.g., the global flame shape, size, or location) suddenly changes from stable to unstable or vice-versa. These stability shifts are driven by changes in the relative A/F ratios in the primary and secondary combustion zones, changes in the gas velocity profile, and/or the rate of mixing between these zones. A typical operating condition for low NOx coal-fired burners involves fuel-rich combustion in the primary zone and fuel-lean combustion in the secondary zone. Primary zone combustion becomes unstable and flickers on and off in repeated ignition and extinction events, when conditions in the primary zone are too rich or the flow velocity is too high. Under extreme conditions, primary zone combustion may be completely extinguished.
Extinction of combustion at the base of the primary zone represents a bifurcation in which the xe2x80x9cattachedxe2x80x9d flame state is no longer stable (i.e., the initial flame front is no longer supported in the vicinity of primary air and fuel exit pipes). When the initial flame front is no longer supported in the vicinity of the fuel exit pipes, the flame front may shift axially downstream from the face of the burner and can assume a detached xe2x80x9cliftedxe2x80x9d condition. A lifted flame represents an alternate stable flame state that can persist even though the attached flame is unstable. In a lifted flame, the distance from the burner face to the flame boundary and the stability of that boundary depends on many factors such as the primary air exit velocity, the A/F ratio in the secondary zone and the detailed air flow velocity profile. Under some conditions, stable lifted and attached flame states may co-exist, so that the burner can assume either condition depending on the initial burner state. External perturbations to the burner (e.g., air or fuel flow disturbances) may cause transitions between these two states.
Extinction of combustion in the primary zone can also occur if there is an excessive amount of oxygen present. This can happen in coal-fired burners when the release of volatile matter from the fuel is too slow to keep the gas mixture above the lean flammability limit. Whether caused by high air velocity or excessively rich or lean primary zone conditions, lifted flames are an undesirable operating condition typically associated with excessive emissions of pollutants.
Bifurcations and associated flame front shifting can also occur in the radial direction due to excessively high or low rates of mixing between primary and secondary zones. These types of bifurcations can produce axial shifts in flame shape and symmetry that result in helical and/or side-to-side motions. In some cases, flame size may also undergo large expansion and contraction. Large variations in the amount of visible and infrared light emissions from the flame are observed during such events. Like axial flame shifting, radial flame shifts are associated with excessive emissions of pollutants and reduced fuel utilization. As is well known to those of skill in the art, an optimal flame diameter exists. Larger or smaller flame diameters are usually detrimental to performance.
Conventional analysis methods such as Fourier analysis and univariate statistics are based on assumptions that are not entirely valid for burners. Specifically, Fourier analysis assumes that the described processes are linear (i.e., processes in which the observed behavior is produced by superposition of simple modes), while univariate statistics assumes that each event is random and independent from events at other times (i.e., there is no time correlation). When these assumptions are incorrect the results from Fourier analysis and univariate statistics can provide either misleading results or results that are insensitive to real differences (M. J. Khesin et al., xe2x80x9cDemonstration Tests of New Burner Diagnostic System on a 650 MW Coal-Fired Utility Boiler,xe2x80x9d American Power Conference, Chicago, Ill., Volume 59-1, 1997; Krueger et al., xe2x80x9cIllinois Power""s On-Line Operator Advisory System to Control NOx and Improve Boiler Efficiency: An Update,xe2x80x9d American Power Conference, Chicago, Ill., Volume 59-1, 1997; Adamson, et. al., xe2x80x9cBoiler Flame Monitoring Systems for Low NOx Applicationsxe2x80x94An Update,xe2x80x9d American Power Conference, Chicago, Ill., Volume 59-1, 1997; Khesin, M., et. al., xe2x80x9cApplication of a Flame Spectra Analyzer for Burner Balancing,xe2x80x9d presented at the 6th International ISA POWID/EPRI Controls and Instrumentation Conference, June 1996, Baltimore, Md.)
Chaos theory (especially, symbol sequence techniques and temporal irreversibility) avoids the assumptions of conventional analytical methods and thus may provide information unavailable from these well-known techniques. Chaos theory is a prominent new approach for understanding and analyzing deterministic nonlinear processes, which provides specific tools for detecting and characterizing fluctuating unstable patterns of these processes (Gleick, xe2x80x9cChaos: Making a New Science,xe2x80x9d Viking Press, New York, 1987; Stewart, xe2x80x9cDoes God Play Dice? The Mathematics of Chaos,xe2x80x9d Basil Blackwell Inc., New York, 1989; Strogatz, xe2x80x9cNonlinear Dynamics and Chaos,xe2x80x9d Addison-Wesley Publishing Company, Reading, Mass., 1994; Ott et al., xe2x80x9cCoping with Chaos,xe2x80x9d John Wiley and Sons, Inc., New York, 1994; Abarbanel, xe2x80x9cAnalysis of Observed Chaotic Data,xe2x80x9d Springer, N.Y., 1996). Chaos theory has been applied to feedback systems and burner flame analysis (Wang et al. U.S. Pat. No. 5,404,298; Jeffers, U.S. Pat. No. 5,465,219; Fuller et al., xe2x80x9cEnhancing Burner Diagnostics and Control with Chaos-Based Signal Analysis Techniques,xe2x80x9d 1996 International Mechanical Engineering Congress and Exposition, Atlanta, Ga., vol. 4, pp 281-291, Nov. 17-22, 1996; J. B. Green, Jr. et al., xe2x80x9cTime Irreversibility and Comparison of Cyclic-Variability Models,xe2x80x9d Society of Automotive Engineers Technical Paper No. 1999-01-0221 (1999). Because combustion is highly nonlinear, analytical techniques derived from chaos theory (especially, symbol sequence techniques and temporal irreversibility) may be particularly useful for burner flame analysis.
Thus, it has become apparent that new apparatus and methods for monitoring the operating states of burner flames are needed. In particular, what is needed is a method and apparatus that can monitor the operating states of individual burners using nonlinear analytical methods such as symbol sequence analysis and temporal irreversibility on a diagnostically meaningful time scale.
The current invention satisfies this and other needs by providing a method and apparatus, which uses symbol sequence techniques and/or temporal irreversibility methods to monitor the operating state of individual burner flames on an appropriate time scale. Both the method and apparatus of the present invention may be used to optimize the performance of burner flames.
In one aspect, the invention provides a method of monitoring the operating state of a burner flame. First, sensor data representing the operating state of a burner flame is obtained. Second, the data is analyzed with symbol sequence techniques and/or temporal irreversibility methods in combination with conventional statistics and Fourier transforms to determine the operating state of the burner flame. In a more specific embodiment, the operating state of the burner flame is changed on the basis of the first two steps above. Preferably, in this embodiment, the operating state of the burner flame is changed to an optimal flame.
In one embodiment, the burner flame is a low-NOx coal flame. In another embodiment, the burner flame is an oil flame.
In one embodiment, the data on the burner flame operating state is further processed. In another embodiment the data is stored. In yet another embodiment, the operating state of the burner flame is communicated to a display.
Preferably, a sensor is used to obtain a data on the operating state of the burner. More preferably, the sensor is an optical scanner. In one embodiment, the scanner is an infrared scanner. In another embodiment, the sensor is a pressure transducer or an acoustical scanner.
Preferably, the operating state of the burner flame is converted to a sequence symbol histogram. In one embodiment, the symbol sequence histogram is further stored. In another embodiment, the symbol sequence histogram is compared with a library of symbol sequence histograms to determine the operating state of the burner flame. In one embodiment, the temporal irreversibility function is a time delay function, a time delay and symbolic function or a symbolic function.
In one embodiment, the operating state of the burner flame is an edge lifting flame. In another embodiment, the operating state of the burner flame is a sporadic lifting flame. In still another embodiment, the operating state of the burner flame is an unsteady fuel feed flame. In still another embodiment, the operating state of the burner flame is an optimal flame.
In one embodiment, the operating state of the burner flame is correlated to the total A/F ratio of the burner flame. In another embodiment, the operating state of the burner flame is correlated to the primary air/coal ratio of the burner flame.
In a second aspect, the present invention provides an apparatus for monitoring the operating state of the burner flame. The apparatus has a sensor that provides data on the operating state of the burner flame, which is coupled to a computer that performs symbol sequence analysis on the data to determine the operating state of the burner flame. The computer may also calculate a temporal irreversibility function from the data. Preferably, the temporal irreversibility function is a time delay function, a time delay and symbolic function or a symbolic function. In a preferred embodiment, the apparatus is coupled to an existing control unit (traditional distributed control system (DCS) or neural-network-based control system or a combination of both) that can change the operating state of the burner flame.
In one embodiment, the apparatus has a display coupled to the computer that exhibits the operating state of the burner flame. In another embodiment, the apparatus has a data processor coupled to the computer. In yet another preferred embodiment, the apparatus has a data storage unit coupled to a computer.
In one embodiment, the burner flame is a low-NOx coal flame. In another embodiment, the burner flame is an oil flame.
Preferably, the sensor is an optical scanner. In one embodiment, the scanner is an infrared scanner. In another embodiment, the sensor is a pressure transducer or an acoustical sensor.
Preferably, the apparatus of the invention converts the operating state of the burner flame to a sequence symbol histogram. In one embodiment, the symbol sequence histogram is stored. In another embodiment, the symbol sequence histogram is compared with a library of symbol sequence histograms to determine the operating state of the burner flame.
In one embodiment, the operating state of the burner flame is an edge lifting flame. In another embodiment, the operating state of the burner flame is a sporadic lifting flame. In still another embodiment, the operating state of the burner flame is an unsteady fuel feed flame. In still another embodiment, the operating state of the burner flame is an optimal flame.
In one embodiment, the operating state of the burner flame is correlated to the total A/F ratio of the burner flame. In another embodiment, the operating state of the burner flame is correlated to the primary air/coal ratio of the burner flame.
In one embodiment, weighting factors are applied to some or all of the analyses including conventional statistics, temporal irreversibility and symbol sequence to produce an overall assessment of the operating state of the burner. This overall assessment is stored as a library function to which future assessments can be compared to both qualitatively and quantitatively describe the operating state of the burner.